A leading coronavirus vaccine candidatemight be made available to some health care workers this fall, the CEO of the company that makes the shottold financiers.

The comment by Moderna CEO Stephane Bancels reflects the urgent need to protect front-line workers against the growing pandemic. But numerous vaccine experts have cautioned about the risks of rushing a vaccine to the public.

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By the fall the FDA would at best have only initial data on the vaccine’s safety and its ability to produce antibodies and shape an immune response. The vaccine by then might have been tested in a few hundred people who probablywouldn’t have been exposed to the disease.

An SEC filing published on Monday quoted Bancels as telling Goldman Sachs officers that while the vaccine is unlikely to be commercially available for at least 12 to 18 months, under an emergency FDA authorization it “could be available to some people, possibly including healthcare professionals, in the fall of 2020.”

FDA’s decision to provide an emergency use authorization could depend on the severity of the coronavirus outbreak at that time, the proportion of hospitalized or dying patients, and any vaccine safety and efficacy data that's available, Penny Heaton, CEO of the Bill & Melinda Gates Medical Research Institute, told POLITICO.

“They would have to move forward very cautiously,” said Heaton. One risk of vaccinating large numbers of people before a vaccine has been tested thoroughly is “immune enhancement,” in which the vaccine combines with viral components to cause an allergic reaction or a more severe infection.

Immune enhancement has occurred with past vaccines, including other experimental coronavirus vaccines when tested in animals. However, Moderna officials and the NIH scientists they work with say they are confident their vaccine will not prompt this side effect.

Moderna created its gene-based vaccine in the laboratory using synthetic chemistry methods that allow it to move extremely fast. However, its technology has not been tested in any epidemic, and there is currently no licensed coronavirus vaccine in the world. A Chinese company using a different technique has also started testing a vaccine, and a few of some 20-35 other vaccine candidates are close on Moderna's heels in the United States and Europe.

At an online meeting hosted by the FDA and the European Medicine Agency last week to discuss joint approaches to coronavirus vaccine, regulators committed to keeping a sharp eye out for immune enhancement, said Peter Marks, FDA’s top regulator for vaccines, in a release.

The company and FDA declined to comment further on Bancel’s statement. The National Institute of Allergy and Infectious Diseases, which is working with Moderna on its vaccine, did not immediately respond to a request for comment.